The broad objective of our work is to understand the neural regulation of the heart under a variety of natural and experimental conditions. The specific objective of the present proposal is to investigate the functional significance of the cardiac and respiratory periodicities observed in both divisions of the autonomic nervous system. With respect to the vagal control of the heart, attention will be directed principally toward extending our knowledge of the phase-related effects of vagal activity. Experiments will be conducted to distinguish between the effects of interstimulus interval and of phase on the following aspects of cardiac function: heart rate, atrioventricular conduction, and the automaticity of certain ectopic pacemaking centers. The phase-related effects of vagal stimulation will be studied over a range of levels of cardiac sympathetic activity and over a range of doses of certain rapidly acting digitalis preparations. The phase-related effects of the aortic and carotid sinus baroreceptor reflexes will also be investigated under control conditions, and at various dose levels of digitalis and clonidine. With respect to the sympathetic control of the heart, the changes in heart rate, A-V conduction, and myocardial contractility during steady sympathetic stimulation will be compared with the effects produced by intermittent stimulation. During the latter mode of stimulation, the grouping of activity will be at cardiac or respiratory frequencies. BIBLIOGRAPHIC REFERENCES: Martin, P.: Depression of atrioventricular sensitivity in the dog by successive brief bursts of vagal stimulation. Circ. Res. 38:448-453, 1976. Levy, M.N.: Parasympathetic control of the heart. In Neural Regulation of the heart, ed. Randall, W.C., New York, Oxford University Press, 1977, pp. 95-129.